1. Field of the Invention
The invention relates to the cleaning of gases to remove toxic impurities, in particular to the cleaning of fumes from power stations before they are expelled into the atmosphere.
It relates more particularly to a process for cleaning a gas to remove oxides of nitrogen and, especially, to remove nitrogen monoxide (NO)
2. Description of the Related Art
The fumes generated by the combustion of gaseous, liquid or solid combustible materials are usually contaminated by oxides of nitrogen originating from the nitrogen in atmospheric air and, in some cases, from nitrogen compounds present in the fuel. In these fumes, nitrogen monoxide (NO) constitutes the majority of the oxides of nitrogen, the remainder consisting principally of nitrogen dioxide (NO.sub.2).
The high toxicity of nitrogen monoxide and of nitrogen dioxide means that they have to be removed from the fumes before the latter are expelled into the atmosphere.
U.S. Pat. No. 4,839,147 (Waagner Biro AG) proposes a process for cleaning fumes to remove oxides of nitrogen, in which an alkali metal sulphite and ammonia gas are introduced into the fumes so as to reduce the oxides of nitrogen to dinitrogen monoxide (N.sub.2 O) which is subsequently reduced to nitrogen by the ammonia. In the case of fumes which simultaneously contain oxides of nitrogen and sulphur dioxide, the alkali metal sulphite is formed in situ in the fumes by introducing an alkali metal bicarbonate or carbonate into these fumes.
The disadvantage of this known process is that it requires the use of a number of reagents, including a gaseous reagent (ammonia) whose toxic nature implies considerable safety measures.